[BPA5150]

Do any of you engineers out there know the effect of ambient air temp on an air cooled bike? Is it as simple as:

Air=20 degrees warmer, Engine=20 degrees warmer, or is there more/less of a correlation between the two?

My bike runs at 3-4 bars whether I'm along the coast in 75 degree air or in the desert with 100 degree air. I can't tell you the exact air temp, because for some UNEXPLAINED reason the M1100 doesn't give air temp like the M696 and M1100s!!!

I also can't tell you the exact oil temp, because Ducati uses a silly bar system. I'd much prefer it just showed the oil temp numerically...

 

[Scissors]

Well, there's actual average engine temperature, and then there's measured temperature. The only thing your display gives you is the measured temperature at the oil temperature gauge (which is really more of a head temperature gauge), which is located on the front side of the rear cylinder.

This particular spot will run up to a minimum temperature simply because it is sheltered from a great deal of direct cooling. The oil serves as a coolant by "taking" heat from hotter locations and "depositing" it in cooler locations as it travels through the engine and oil cooler.

My air-cooled Yamaha measures both oil and head temperatures, but its oil temperature measurement is after passing through the oil cooler, whereas the head temperature measurement is on one of the jugs. I've found it to very consistently be a 60 degree difference between the two, but have found little correlation between air temperature and these other temperatures. A drop of 80 degrees in air temperature might yield a drop of 20 degrees in running temperature.

In the Yamaha's case, it runs pretty consistently at 180/240 degrees during the coldest days in winter versus 200/260 degrees during the same driving conditions in the heat of summer.

This may, in part, be because something like 60% of cooling is accomplished via the exhaust, which is not affected by ambient air temperatures. The remaining cooling is done in the form of convection and radiation--that is, through heating of surrounding air, and by giving off infrared radiation. The radiation portion of this remaining 40% also has nothing to do with air temperature.

That convection through surrounding (cooling) air is only part of 40% of cooling (give or take) pretty much explains why it isn't a 1-for-1 link between air temperatures and engine temperatures.

(I should also note that initially, when the engine is cold, a lot of cooling is in the form of conduction as the heat is transferred to the cool, surrounding materials. Once up to operating temps all conduction does is carry heat to other components which then shed heat via convection and radiation.)

 

[Scissors]

Summary:
Heat is lost in three ways in an engine which is at operating temps:

As radiation, which is not affected by ambient air temperatures.
As convection via hot exhaust, which is not affected by ambient air temperatures.
As convection via the air passing over the oil cooler, fins, and attached physical components, which is affected by ambient air temperatures.

 

[Gilles]

Summary:
Heat is lost in three ways in an engine which is at operating temps:

As radiation, which is not affected by ambient air temperatures.
As convection via hot exhaust, which is not affected by ambient air temperatures.
As convection via the air passing over the oil cooler, fins, and attached physical components, which is affected by ambient air temperatures.

I'll give you a 9/10 on that.

The 2nd point "as convection via hot exhaust" is actually thermodynamic heat sink loss that even a perfect (Carnot) engine loses. Not convection but a loss of entropy :abduct: every engine causes when converting heat to motive power.

 

[BPA5150]

So ambient air doesn't make a heck of a lot of difference, less than the actual air temp difference?

Like I said, my bikes seems to run the same (I don't get exact numbers, just "bars") whether it's 70 degrees or 100 degrees outside.

Bottom line is I guess I don't really need to worry more about over-heating on hot summer days.

Great info, thanks for the responses.